macroevolution Long-term, large-scale evolutionary changes among groups of species. Compare microevolution.

Macronutrients Chemical elements that organisms need in large amounts to live, grow, or reproduce. Examples are carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, and iron. Compare micronutrients.

magma Molten rock below the earth's surface.

Malnutrition Faulty nutrition, caused by a diet that does not supply an individual with enough protein, essential fats, vitamins, minerals, and other nutrients needed for good health. See kawashiorkor, maramus. Compare overnutrition, undernutrition.

mangrove swamps Swamps found on the coastlines in warm tropical climates. They are dominated by mangrove trees, any of about 55 species of trees and shrubs that can live partly submerged in the salty environment of coastal swamps.

mantle Zone of the earth's interior between its core and its crust. See lithosphere. Compare core, crust .

manufactured resources Manufactured items made from natural resources and used to produce and distribute economic goods and services bought by consumers. These include tools, machinery, equipment, factory buildings, and transportation and distribution facilities. Also used manufactured capital. Compare financial resources, human resources, natural resources.

marasmus Nutritional deficiency disease caused by a diet that does not have enough calories and protein to maintain good health. See kwashiorkor, malnutrition.

market price equilibrium point State in which sellers and buyers of an economic good agree on the quantity to be produced and the price to be paid.

mass depletion Period of species loss in which extinction rates are much higher than normal but not high enough to classify as a mass extinction. Compare background, extinction, mass extinction.

mass extinction Catastrophic, widespread, often global event in which major groups of species are wiped out over a short time compared with normal (background) extinctions. Compare background extinction, mass depletion.

mass transit Buses, trains, trolleys, and other forms of transportation that carry large numbers of people.

material efficiency Total amount of material needed to produce each unit of goods or services. Also called resource productivity. Compare energy efficiency.

matter Anything that has mass (the amount of material in an object) and takes up space. On the earth, where gravity is present, we weigh an object to determine its mass.

matter-recycling economy Economy that emphasizes recycling the maximum amount of all resources that can be recycled. The goal is to allow economic growth to continue without depleting matter resources and without producing excessive pollution and environmental degradation. Compare high-throughput economy, low-throughput economy.

mature community Fairly stable, self-sustaining community in an advanced stage of ecological succession; usually has a diverse array of species and ecological niches; captures and uses energy and cycles critical chemicals more efficiently than simpler, immature communities. Also climax community is used. Compare immature community.

median lethal dose example: LD50 Amount of a toxic material per unit of body weight of test animals that kills half the test population in a certain time.

megacity City with 10 million or more people.

meltdown Melting of the core of a nuclear reactor.

mesosphere Third layer of the atmosphere; found above the stratosphere. Compare stratosphere, troposphere.

mesotrophic lake Lake with a moderate supply of plant nutrients. Compare eutrophic lake, oligotrophic lake.

metabolism Ability of a living cell or organism to capture and transform matter and energy from its environment to supply its needs for survival, growth, and reproduction. 

metamorphic rock Rock produced when a preexisting rock is subjected to high temperatures (which may cause it to melt partially), high pressures, chemically active fluids, or a combination of these agents. See rock cycle. Compare igneous rock, sedimentary rock.

Microclimates Local climatic conditions that differ from the general climate of a region. Various topographic features of the earth's surface such as mountains and cities typically create them.

microevolution Small genetic changes a population undergoes. Compare macroevolution.

Micronutrients Chemical elements organisms need in small or even trace amounts to live, grow, or reproduce. Examples are sodium, zinc, copper, chlorine, and iodine. Compare macronutrients.

microorganisms Organisms such as bacteria that are so small they can be seen only by using a microscope.

midsuccessional plant species Grasses and low shrubs that are less hardy than early successional plant species. Compare early successional plant species, late successional plant species.

mineral Any naturally occurring inorganic substance found in the earth's crust as a crystalline solid. See mineral resource.

mineral resource Concentration of naturally occurring solid, liquid, or gaseous material in or on the earth's crust in a form and amount such that extracting and converting it into useful materials or items is currently or potentially profitable. Mineral resources are classified as metallic (such as iron and tin ores) or nonmetallic (such as fossil fuels, sand, and salt).

minimum dynamic area (MDA) Minimum area of suitable habitat needed to maintain the minimum viable population. See minimum viable population.

minimum viable population (MVP) Estimate of the smallest number of individuals necessary to ensure the survival of a population in a region for a specified time period, typically ranging from decades to 100 years.

model Approximate representation or simulation of a system being studied.

monoculture Cultivation of a single crop, usually on a large area of land. Compare polyculture, polyvarietal cultivation.

multiple use Use of an ecosystem such as a forest for a variety of purposes such as timber harvesting, wildlife habitat, watershed protection, and recreation. Compare sustainable yield.

municipal solid waste Solid materials discarded by homes and businesses in or near urban areas. See solid waste.

Mutagen Chemical or form of radiation that causes inheritable changes (mutations) in the DNA molecules in the genes found in chromosomes. See carcinogen, mutation, teratogen.

mutation Random change in DNA molecules making up genes that can yield changes in anatomy, physiology, or behavior in offspring. See mutagen.

mutualism Type of species interaction in which both participating species generally benefit. Compare commensalism.